Process to recover energy from hot gas

ABSTRACT

A process to recover energy from a gas having a temperature of above 650° C. and an absolute pressure of more than 1.7 bar and having non-solidified alkali containing compounds and particles by performing the following steps:
     (a) cooling the gas to a temperature of below 550° C. by means of a shell-tube heat exchanger, wherein the hot gas is passed at the shell side and coolant water is passed at the tube side, wherein steam is formed, from which steam energy is recovered;   (b) separating the particles from the gas by means of one or more sequentially arranged centrifugal separation devices to a dust level of below 400 mg/Nm 3 ; and   (c) expanding the gas in an expander to recover energy.

Process to recover energy from a gas having a temperature of above 650°C. and an absolute pressure of more than 1.7 bar and comprising bothsolid and not-yet-solidified alkali containing compounds and particles.Such a gas is for example produced in recently developed continuous ironmaking and steel making process, such as the HI smelt process.

Steel is an iron-base alloy containing less than about 1% carbon andcommonly other alloying elements. Steel is presently manufactured fromblast furnace pig iron (“hot metal”), DRI (direct reduced iron) andscrap iron and steel. DRI, also referred to as sponge iron, is producedby solid state direct reduction of iron ore.

The conventional separate unit operations of steel making-batchby-product coke ovens, continuous iron making blast furnaces and batchsteel making furnaces have dominated the industry for the past onehundred years. Aside from important increases in the size and efficiencyof the apparatus employed, there have been only two major changes inthis period: the ubiquitous application of tonnage oxygen to enrich orreplace process air, and the use of agglomerated, heat indurated, highgrade iron mineral concentrates to supplement or replace natural lumpiron ore.

In recent years, for increasingly compelling reasons of burdensomecapital and operating costs, and because of the need for environmentalprotection, there has been a sharp increase in continuous iron makingand steel making process research and development. Such smeltingreduction methods are for example described in U.S. Pat. Nos. 5,891,214,5,759,495 and 5,060,913. The coal-based COREX continuous iron makingprocess operates commercially, but it is dependent on lump iron-richfeed and on a satisfactory market for the large volume of export gas itproduces. Currently, leading infant continuous processes are for exampleprocesses referred to as HIsmelt, DIOS and Romelt (trademarks). All ofthese processes are devoted to iron making, which overcome thedisadvantages of the blast furnace process. These new processes are highintensity, coal-based, in-bath smelting processes treating iron orefines.

The oxygen supplied to HIsmelt is primarily air preheated to 1200° C.Iron ore fines, coal and flux are bottom-injected using nitrogen ascarrier gas. A high velocity, high mass flow, hot air blast is injectedthrough a single top tuyere. The bath is highly turbulent, and the metaland slag produced are separated externally. The relatively short,horizontal smelting furnace is round in cross-section. Its off-gaspasses to a circulating fluidized bed to capture entrained droplets anddust before being further used downstream. The DIOS process comprises acirculating fluidized bed, pre-reduction furnace linked to a smeltingfurnace similar to a tall L-D oxygen converter. Furnace feed consists ofpartially reduced fine iron ore, coal, oxygen, and flux. The furnace isbottom-stirred using nitrogen, and operates at 2 atmospheres gauge. TheRomelt process employs submerged injection of oxygen-enriched air forsmelting of iron ore fines directly introduced with coal into a largevolume, violently splashing fluid slag bath.

The above processes will produce very large volumes of hot gascontaining carbon monoxide, hydrogen, dust and compounds, which areoriginally present in the iron ore and the coal. Examples of suchcontaminants are alkali compounds such as sodium and potassium. Thesecompounds are in a liquid or gaseous state at temperatures of above 775°C. At lower temperatures these alkalis will condensate and subsequentlysolidify onto the surface of process equipment and dust particlespresent in the gas. The alkalis may for example solidify in the form ofNaCl, KCl, Na₂CO3 and K₂CO₃. The formation of such condensing and solidsalts makes it difficult to just simply cool the gas and recover theheat. A method of treating the hot gas is by cooling with evaporatingwater. The advantage of such a method is that the alkali compounds maybe recovered as an aqueous solution before they can cause any fouling ofany downstream process equipment. A disadvantage is that the aqueoussolution, containing also dust and possibly coal particles, has to betreated before it can be disposed into the environment. Furthermore sucha method is not an efficient method of recovering energy from the hotgas.

In U.S. Pat. No. 4,424,766 a hydro-pressurised fluidised bed combustorfor coal combustion is described. A tubular heat exchanger is positionedin the freeboard of the fluidised bed combustor vessel.

In U.S. Pat. No. 6,044,977 an apparatus is described for removingmicroparticulates from a gas. The purified gas is fed out forutilization in driving a gas turbine for electrical power generation orother purpose.

There is thus a need for a process wherein the temperature of the hotgas can be greatly reduced, while the problems associated with thesolidification of the alkali compounds is reduced. The present inventionprovides a process wherein the above-described problems are overcome andenergy is recovered in a more efficient manner.

The following process achieves this object. Process to recover energyfrom a gas, obtained from a smelting reduction process used tocontinuously prepare steel, having a temperature of above 650° C. and anabsolute pressure of more than 1.7 bar and comprising non-solidifiedalkali containing compounds and particles by performing the followingsteps:

-   (a) cooling the gas to a temperature of below 550° C. by means of a    shell-tube heat exchanger, wherein the hot gas is passed at the    shell side and coolant water is passed at the tube side, wherein    steam is formed, from which steam energy is recovered, and wherein    the shell-tube heat exchanger comprises a membrane wall positioned    in an elongated vessel, which elongated membrane wall is open at    either side for gas to enter and leave the inner part of the space    surrounded by said membrane wall, which inner space is provided with    a plurality of heat exchanging tubes, which tubes are interconnected    at their exterior in a group wise manner and positioned in said    inner space such that a plurality of channels for passage of hot gas    exist, which passages run parallel to the elongated walls of the    membrane wall,-   (b) separating the particles from the gas by means of one or more    sequentially arranged centrifugal separation devices to a dust level    of below 400 mg/Nm³,-   (c) expanding the gas in an expander to recover energy.

The hot gas used in step (a) will have a temperature of above 650° C.,especially more than 800° C. The upper temperature may be 1000° C. Thepressure of the hot gas will be above 1.7 and more preferably above 1.9bar absolute (bara). This minimum pressure is required to achieve asufficient energy recovery in step (c). The absolute pressure may be upto 40 bara. The hot gas will contain solid particles. These solidparticles may for example be soot and ash when the hot gas is obtainedin a continuous iron smelt process as described above. The presentprocess is best suited to be used starting with a hot gas comprisingmore than 0.5 g/Nm³ of particles. Preferably the hot gas contains morethan 5 g/Nm³ particles. This is advantageous to achieve a minimumself-cleaning effect of the gas flowing through the shell-tube heatexchanger of step (a). There is no practical upper limit to the amountof particles present in the hot gas. Suitable hot gasses as obtained inthe above referred to continuous iron making processes will usually havea content of particles of less than 100 g/Nm³.

The hot gas will also comprise alkali compounds. Typical examples ofnon-solidified alkalis are sodium and potassium. The content of sodiumis preferably between 0.02–0.08 vol % and the content of potassium ispreferably between 0.02–0.1 vol %. The hot gas may also contain carbonmonoxide and hydrogen if the hot gas is obtained under not completelycombustion conditions. The content of carbon monoxide may be between 10and 30 vol % of the hot gas. The hydrogen content may be between 5 and15 vol %. An example of a hot gas having the above composition is theflue gas as obtained in the above referred to smelting reductionprocesses, as for example the COREX, HIsmelt, DIOS and Romelt process.

It has been found that by using the shell-tube heat exchanger of step(a) a sufficient temperature reduction is possible while at the sametime fouling of the heat exchanger, due to solidification of alkalis, isavoided. Fouling is avoided as much as possible because the gas flows atthe shell side of the heat exchanger. The shell-tube heat exchanger ispreferably designed having a relatively high heat-exchanging surface. Inuse the gas will flow at a relatively low gas velocity through the shellside of the heat exchanger. It has been found that part of the foulingis removed from the surfaces of the heat exchanger by the self-cleaningpower from the particles present in the hot gas. Nevertheless somefouling is expected to occur and therefore the surface of the heatexchange tubes will have to be cleaned by preferably mechanical rappers.Examples of such rappers are described in DE-A-2710153 and EP-A-254379.

The shell-tube heat exchangers comprise a membrane wall having forexample a tubular or rectangular box like form. The membrane wall ispreferably positioned in an elongated vessel. The tubes of the membranewall preferably run parallel to the elongated side of said wall. Theelongated membrane wall is open at either side for gas to enter andleave the inner part of the space surrounded by said membrane wall. Thisinner space is provided with a plurality of heat exchanging tubes. Thesetubes are interconnected at their exterior in a group wise manner andpositioned in said inner space such that a plurality of channels forpassage of hot gas exist. These passages run preferably parallel to theelongated walls of the membrane wall. For example when a tubularmembrane wall is used the inner tubes may be arranged in a plurality ofconcentric tubular formed groups of spiral tubes. The tubes of onetubular group are suitably interconnected. The passages for hot gas willbe the annular spaces between said tubular groups of tubes. When anelongated rectangular box like membrane wall is used the groups ofinterconnected tubes may be flat walls of tubes positioned parallel inthe box like space. The passage for hot gas will then have an elongatedbox like shape. Preferably each group of tubes and the membrane wall isprovide with a separate rapper means. Because the tubes of eachindividual group of tubes are inter-connected the number of rapper meansto clean each group can be limited.

Cooling water preferably runs counter-current through the tubes in thedifferent groups and through the tubes of the membrane wall runs withthe hot gas. Groups of tubes may also be used to further heat saturatedsteam to obtain super heated steam.

Examples of suitable heat-exchanger which can find application in step(a) are described in EP-A-342767. More preferably a heat-exchanger isused wherein the above referred to gas passages are arranged in such amanner that, in operation, the velocity of the gas flowing through thesaid gas passages, is kept substantially constant. It has been foundthat there is only a small gas velocity range wherein the gas has asufficient self-cleaning effect to reduce fouling at the one hand and aminimal equipment erosion effect on the other hand. By reducing thecross-sectional area of the gas passages in the heat-exchanger in thedownstream direction a substantially constant gas velocity can bemaintained in said passages. An example of a preferred heat exchangerhaving such reduced gas passages is described in EP-A-722999, whichpublication is incorporated herein by reference.

In step (a) the temperature is reduced to a temperature below 550° C.and preferably below 520° C. Because at these low temperatures mostnon-solidified alkalis are present as solids it is not necessary toreduce the temperature to very low levels. From an energy recoveryviewpoint it is preferred that the temperature of the gas leaving step(a) is at least 500° C. From the steam or optionally super heated steamenergy can be recovered by means of a steam turbine.

In step (b) solid particles are removed from the gas by means of one ormore sequentially arranged centrifugal separation devices to a dustlevel of below 400 mg/Nm³. These solid particles will comprisesolidified alkali compounds and the dust which was originally present inthe hot gas. The dust level of the gas as obtained in step (b) ispreferably lower than 350 mg/Nm³ and more preferably lower than 280mg/Nm³. In addition to this requirement the amount of coarse dust,particles having a mean diameter of more than 10 microns, is preferablyless than 5 and more preferably less than 2 mg/Nm³. The dust levelsneeds to be lowered in step (b) to prevent erosion of the expansionturbine as used in step (c).

The centrifugal separator which is preferably used in step (b) can beany known separator which separates solids from a gas by making use ofcentrifugal forces and which claims to reduce the level of dust to thedesired level. Preferably the separation is performed by means of acyclone separator in step (b), more preferably by means of a so-calledaxial entry cyclone. Such cyclone comprise two concentric tubes, theinner tube serving as a gas outlet and vortex finder and the outer tubeserves as a swirl chamber in which the particles are centrifugal heldagainst the wall and away from the vortex. The tangentially velocity isimpaired to the gas feed by means of swirl vanes located between theinner and outer tube. The inner tube protrudes partly the outer tubefrom above. Solids are removed at the lower end of the outer tube.Preferably the separator comprises a plurality of such tubes operatingin parallel. Examples of such separators are well known and aredescribed in for example GB-A-1411136. A commercial example is the ShellThird stage separator as for example described in HydrocarbonProcessing, January 1985, pages 51–54. Variations of such separators areshown as a figure in Perry (see below) in FIG. 20.98. If the level ofparticles in the hot gas leaving step (a) is more than 1 g/Nm³ andespecially more than 10 g/Nm³ a pre-separation is preferably performedbefore the gas is fed to a separator as described above. Such a roughseparation is preferably performed by means of a standard tangentialinlet cyclone as for example described in Perry's Chemical Engineers'handbook, 5th edition, 1973, McGraw-Hill Inc., page 20–83 to 20–85. Thelevel of particles is preferably reduced to below 1 g/Nm³.

In a preferred embodiment part or all of the relative coarse particles,which may comprise combustionable material and which are separated fromthe gas in the above described rough separation of step (b), arerecycled to the process, especially the above referred to smeltingreduction processes, which generates the hot gas. The smaller particles,as separated in the final separation step of step (b), for example bymeans of the Shell Third Stage Separator, will contain relatively morealkali deposits than the coarse particles. Advantageously these smallerparticles are not recycled to said process. Thus a process is obtainedwherein no build-up of alkalis will occur while the net amount of solidsbeing produced by the process in step (b) is minimized.

In step (c) the gas stream is passed into a power recovery expander anddepressurized, with the energy recovered from the gas stream being usedfor useful work such as driving a compressor or generating electricity.A bypass system, which diverts the gas stream around the power recoveryexpander, will normally be employed to prevent over speeding of theexpander. These systems are described in for example U.S. Pat. Nos.3,777,486 and 3,855,788. The power recovery expander and the otherequipment required to practice the invention are rather specialized, butare available commercially.

If the feed gas of the process according to the present inventioncomprises carbon monoxide an additional step (d) is preferablyperformed. Step (d) comprises the combustion of the carbon monoxide tocarbon dioxide. The combustion of CO-containing gas is usually performedunder controlled conditions in a separate so-called CO-boiler orcombustion device enriched with air and continuously fed withCO-containing gas. The CO-boiler can be equipped to accept at least oneother fuel, which is used in start-up, or more commonly to supplementthe fuel value of the flue gas. Such processes are well known. Otherexamples are described in U.S. Pat. No. 2,753,925 wherein the releasedheat energy from CO-containing gas combustion is employed in thegeneration of high-pressure steam.

FIG. 1 shows a preferred embodiment of the present invention. FIG. 1shows an smelting reduction process reactor (1) to which coal, iron ore(2) and oxygen containing gas (3) is fed. Iron is recovered via (4) anda flue gas (5) is produced. The hot flue gas is led via overhead conduit(5) and via a shell-tube heat exchanger (6), a rough cut cyclone (7) toa vessel (8) comprising a plurality of axial entry cyclone separators(9). In heat-exchanger (6) steam is produced and discharged via (10) toan energy recovery facility, which may be an steam turbine. Theparticles separated in rough cut cyclone (7) are recycled to reactor (1)via (11). The fine, alkali containing, particles separated in vessel (8)are discharged via (12). The hot gas, poor in solids, is fed to expander(13) to produce energy (E). The gas comprising carbon monoxide is fed toa CO boiler (14) wherein energy (E) is recovered in (15).

1. A process to recover energy from a hot gas, obtained from a smeltingreduction process used to continuously prepare steel, wherein the hotgas has a temperature of above 650° C. and an absolute pressure of morethan 1.7 bar and comprising non-solidified alkali containing compoundsand particles by performing the following steps: (a) cooling the hot gasto provide a cooled gas having a temperature of below 550° C. by meansof a shell-tube heat exchanger, wherein the hot gas is passed at theshell side of the shell-tube heat exchanger and coolant water is passedat the tube side of the shell-tube heat exchanger, wherein steam isformed, from which steam energy is recovered, and wherein the shell-tubeheat exchanger includes an elongated membrane wall formed by a pluralityof elongated tubes connected together so as to form the elongatedmembrane wall, wherein the elongated membrane wall further defines atubular space having at one end of the tubular space an inlet openingfor receiving the hot gas into the tubular space and at an opposite endof the tubular space an outlet opening for discharging the cooled gasfrom the tubular space, wherein a plurality of heat exchanging tubesproviding the tube side of the shell-tube heat exchanger that passthrough the tubular space thereby defining a plurality of channels forthe passage of the hot gas within the tubular space and between theplurality of heat exchanging tubes, (b) separating the particles fromthe cooled gas by means of one or more sequentially arranged centrifugalseparation devices to a dust level of below 400 mg/Nm3 to give a reduceddust gas, (c) expanding the reduced dust gas in an expander to recoverenergy.
 2. The process according to claim 1, wherein the hot gas used instep (a) has a temperature of above 800° C.
 3. The process according toclaim 2, wherein the hot gas contains more than 5 g/Nm³ of particles. 4.The process according to claim 3, wherein the hot gas contains between0.02–0.08 vol % sodium and between 0.02–0.1 vol % potassium.
 5. Theprocess according to claim 4, wherein the content of carbon monoxide isbetween 10 vol % and 30 vol % in the hot gas and the hydrogen content inthe hot gas is between 5 vol % and 15 vol %.
 6. The process according toclaim 5, wherein the plurality of channels is arranged in such a mannerthat, in operation, the velocity of the hot gas flowing through theplurality of channels is kept substantially constant.
 7. The processaccording claim 6, wherein the temperature of the hot gas is reduced instep (a) to provide the cooled gas having a temperature between 500° C.and 520° C.
 8. The process according to claim 7, wherein the dust levelof the reduced dust gas as obtained in step (b) is lower than 280mg/Nm³.
 9. The process according to claim 8, wherein the content ofparticles having a mean diameter of more than 10 microns in the reduceddust gas as obtained in step (b) is less than 5 mg/Nm³.
 10. The processaccording to claim 9, wherein the separation in step (b) is performed bymeans of an axial entry cyclone.
 11. The process according to claim 10,wherein in step (b) a pre-separation is performed if the level ofparticles in the cooled gas leaving step (a) is more than 1 g/Nm³ andwherein said pre-separation is performed in a tangential inlet cycloneseparator.
 12. The process according claim 11, wherein the hot gas isobtained in a smelting reduction process and the material which isseparated in said pre-separation is recycled to said smelting reductionprocess.
 13. The process according to claim 12, wherein a step (d) isperformed when the reduced dust gas as obtained in step (c) comprisescarbon monoxide and hydrogen, said step (d) comprising the combustion ofthe carbon monoxide to carbon dioxide.
 14. The process according toclaim 1, wherein the hot gas comprises more than 0.5 g/Nm³ of particles.15. The process according to claim 1, wherein the hot gas containsbetween 0.02–0.08 vol % sodium and between 0.02–0.1 vol % potassium. 16.The process according to claim 1, wherein the content of carbon monoxideis between 10 vol % and 30 vol % in the hot gas and the hydrogen contentin said hot gas is between 5 vol % and 15 vol %.
 17. The processaccording to claim 1, wherein the plurality of channels is arranged insuch a manner that, in operation, the velocity of the hot gas flowingthrough the plurality of channels is kept substantially constant. 18.The process according to claim 1, wherein the temperature of the hot gasis reduced in step (a) to provide the cooled gas having a temperaturebetween 500° C. and 520° C.
 19. The process according to claim 1,wherein the dust level of the reduced dust gas as obtained in step (b)is lower than 280 mg/Nm³.
 20. The process according to claim 1, whereinthe content of particles having a mean diameter of more than 10 micronsin the reduced dust gas as obtained in step (b) is less than 5 mg/Nm³.21. The process according to claim 1, wherein the separation in step (b)is performed by means of an axial entry cyclone.
 22. The processaccording to claim 1, wherein a step (d) is performed when the reduceddust gas as obtained in step (c) comprises carbon monoxide and hydrogen,said step (d) comprising the combustion of the carbon monoxide to carbondioxide.
 23. An energy recovery process, said process comprises:providing a hot gas generated by an iron smelting reduction process andhaving a hot gas temperature above 650° C. and a pressure above 1.7 bar,and wherein said hot gas contains a non-solidified alkali compound andsolid particles at a solids concentration of more than 0.5 g/Nm³;cooling said hot gas by use of a shell-tube heat exchanger having ashell side and a tube side by passing said hot gas through said shellside of said shell-tube heat exchanger and passing cooling water throughsaid tube side of said shell-tube heat exchanger and yielding cooled gasfrom said shell side of said shell-tube heat exchanger and steam fromsaid tube side of said shell-tube heat exchanger, wherein said cooledgas has a cooled gas temperature that is below 550° C. and contains asolidified alkali compound; passing said cooled gas to a centrifugalseparator whereby said solid particles and said solidified alkalicompound are removed from said cooled gas to yield a reduced dust levelgas having a dust level below 400 mg/Nm³; and recovering energy fromsaid reduced dust level gas by expanding said reduced dust level gasthrough an expander turbine.
 24. An energy recovery process as recitedin claim 23, wherein said cooled gas temperature is at least 500° C. 25.An energy recovery process as recited in claim 24, wherein said dustlevel is lower than 350 mg/Nm³.
 26. An energy recovery process asrecited in claim 25, wherein said dust level is such that less than 5mg/Nm3 of the particles in said reduced dust level gas have a meandiameter of more than 10 microns.